Exterior Gateway Protocol (EGP)
The Exterior Gateway Protocol (EGP) was the first inter-AS communications method used with TCP/IP and is still in use today in some circumstances. Rather than allowing your AS border router to locate neighbors on its own, you tell it up front the IP addresses or fully qualified domain names of other AS border routers with which it will exchange routing information. After you do this, EGP takes over to acquire those neighbors.
When an EGP-based AS border router comes alive, it is considered to be in an Acquisition state (EGP has a number of designations called “states” that describe the AS border router’s status, where the terms often double as commands). The state can be different for each acquired neighbor at any given time. Available states in EGP are:
Acquisition. Periodically attempts to acquire new EGP neighbors.
Cease. Periodically lets neighbors know that this router is down, and when all acknowledge the message, transitions to Idle.
Idle. Stores no routing information and only responds to commands that request that it return to one of the more active states.
The router’s state determines what commands it will accept and respond to. When this machine learns of a new neighbor, it sends a Request message—and continues to send them regularly. This message contains the following information:
The sequence number assigned to the Request. All Request packets contain a sequence number, which is uniquely tracked only for a specific AS border router sending to another specific one.
How often Hello messages come. We’ll get to these in a moment. They are not the same HELLO’s that OSPF uses. Default is 30 seconds.
How often Poll messages come. We’ll get to these in a moment as well. Default is 60 seconds.
How long to wait before giving up on the other machine. If the AS border router does not hear from its neighbor within this time period, it declares it unavailable. Default is 120 seconds.
The neighbor sends a Confirm in response, verifying that it is up and running and accepts your AS border router as a neighbor. The AS border router then sets up a small data list including what was just discussed in the preceding list, plus the following:
The sequence number assigned to the Confirm. All Confirm packets contain a sequence number, just as the Request packets do.
The mode this AS border router is in relative to the machine in question. This router might either be in active or passive mode at any moment. The mode determines how the router responds to Hello and Poll messages. If the router is active, then whenever a Hello or Poll arrives it responds with one of the following message types: Confirm, I-Heard-You (IHU), or Update. A passive router might respond to one of these by entering the Up state, which returns the router to participating fully with its EGP neighbors.
The time the Confirm message arrived. This is used to set the zero point for three different variables. The first counts how long until the next time this machine sends a Request to the AS border router in question. The second counts how long the machine waits before sending a Poll. The third maintains a countdown for how much time transpires since it last heard from the machine in question—in case it is silent too long.
Your router now changes to the Down state. This state allows for handling the commands Cease, Hello, and Request.The new ones here are Cease and Hello. An EGP router enters a Cease state in regard to a specific neighbor if it receives a Stop command from the administrator. In response to this command, the machine first enters the Cease state, and then sends a Cease command out to the specific neighbor it is ending its relationship with for the moment.
There are a couple of different things that can happen at this point:
The neighbor receives the Cease command. If the neighbor is up and running just fine, it receives and understands the Cease command. In this case, the neighbor sends a Cease-ack command to your AS border router, and then changes to the Idle state, ignoring anything but a new Request command from the specific neighbor.
The Cease command does not reach the neighbor. If there are network problems, or the neighbor is out of commission, the Cease command may be lost in the “great networking bit bucket.” In this case, the EGP AS border router remains in the Cease state. The router will continue to occasionally send the Cease command until it gets a response. The administrator can issue a second manual Stop command to override waiting for a Cease-ack and immediately send the machine into Idle state relative to this neighbor.
Now, onto the Hello messages and Poll commands.When an EGP AS border router receives a Hello message, it sends back an I-Heard-you (IHU or I-H-U) message, verifying that it is up and running and received the Hello message. Then there’s Poll. Remember that when an EGP AS border router receives a Request command it, sets how often Poll commands are sent to the requesting neighbor and then starts the timer that counts the number of seconds till the next time it needs to send the Poll command.
When the timer reaches the interval value, the machine sends a Poll command to the specified neighbor.When the neighbor receives the Poll command, the response depends on what state and mode this neighbor is in. An EGP AS border router ignores Poll commands if it is in the Down state or Passive mode. If it is not in this state or mode, the neighbor responds with an Update. It is this message that transmits actual information about the networks available in each autonomous system. The data contained in an Update consist of:
All gateways within the AS. Remember that gateways are devices that connect two or more networks.
All gateways the router knows about outside of the AS. This allows for a more complete routing table with less querying.
All networks available through each gateway. The AS border router’s neighbors need to know where to send information.Your router doesn’t need to do as much “thinking” if the neighbors already know to send data directly to a specific gateway for a specific network.
How many routers and gateways data must pass through to reach each specific network from your AS border router. This information allows for shortest path calculations internally within the AS.
Each neighbor’s current state. This information ensures that each of the routers know what kind of data each is expecting from the other.
As you can see, EGP has quite a job to do. Without some kind of method for AS border routers to share information, there would have to be a centralized list of where to send data for all specific networks. The program under Linux that implements this protocol is the unicast version of gated.